The Ta-03s is currently the only all tube amp in the Xduoo line. All of the other currently available Xduoo tube Amps use a tube preamp and a transistor power amp following it. The Ta-03s uses a 12au7 tube preamp followed by a pair of 6s19p power tubes. We discussed the 12au7 tube in installment 2 of this series so some of what follows will be a repeat of some of that information, but be aware that the Ta-03s is enough different sounding that favorites in installment 2 may not be favored in this use case. Here we will discuss the 12au7 as it applies to the Ta-03s and the 6s19p power tubes used in it as well. While there are probably over 100 options for different 12au7 variants, the 6s19P has considerably fewer options with most being either current Chinese manufacture or Russian made surplus tubes from military stockpiles. Luckily what stock is available can usually be had quite inexpensively.
For the backstory and intro to the 12au7 tube, look here.
The 6s19p is a small power triode tube developed in Russia for use as a regulator tube in voltage stabilizers. Dimensionally, it is about an inch taller than the 12au7 seen to the right above. The tube differs from a lot of small tubes in that it uses an indirectly heated cathode and the heaters are visible (vertical glowing red line down the center of the tube) giving the tube a distinct look when compared to the more common directly heated cathode style tubes. The 6s19p is similar to the US made 7233 tube but not an exact match and not pin compatible. The good news is almost every piece of Russian military hardware had some need for voltage regulation so they made a ton of these tubes. The bad news, none of them were originally intended as audio amplifiers and many were never tested for micro-phonics. Tubes are most commonly called the 6S19P in the west, but may be found labeled 6C19, 6S19, 6C19P, 6c19N, 6s19P-V, or 6C19PI or the Cyrillic equivalent.
When purchasing these, be careful as their is a similar numbered 6Ц19П that is often translated as either 6ts19p or 6c19 that is a damper diode designed to reduce voltage spikes and is not the same tube. Visually the 6ts19p is easy to identify as it has a connection at the top of the tube as well as the 9 pin base.
In researching the tube, I found that the orginal Russian 6s19p was primarily made by Svetlana of St. Petersburg, Russia and by Ulyanov of Ulyanovsk, Russia. Makers are easily recognized by either the Winged C logo on Svetlana made tubes or the stylized arrow logo on Ulyanov made tubes. Sovtek tubes have a stylized V logo and were made in the Ulyanov plant as well so while branding is different, construction is the same. I have seen comments suggesting the 6s19p was once made at the Reflector plant in Saratov, Russia as well. These would have a horizontal diamond logo on them and likely be bulk packed for military use. I was unable to find a single pair of the Reflector made tubes for sale at the time I was putting this together but am still looking. I purchased several pairs that claimed to be Reflector made through ebay but all have Winged C logos and most are 1960s produced. I checked with New Sensor that markets Reflector made tubes in the US but they currently catalog only Sovtek branded Ulyanov version. (If you know of available sets, shoot me an email).
Later Chinese tubes seem to have all come from the Shuguang factory as I have not been able to locate samples with any other markings. I did get two additional sets of the Chinese made tubes to make sure I didn’t get a single bad sample when comparing and all performed equally after initially settling down. The markings on the Chinese made tubes are seen in the right most picture below.
This brings up a point worth noting, the 6s19p needs some burn in time to settle down. All tubes need at least an hour to heat up, but these need more like 100 hours of run-time to properly settle in and produce their best sound. I often found the sound to be very grainy when rolling in new tubes. Once given some time, that becomes markedly less pronounced and eventually fades out altogether. For that reason, this article was delayed substantially from the 1st two installments as each tube set took nearly a week worth of burn in before commencing with listening tests. The amp was left running 24/7 with a pair of BeyerDynamic 770s attached. I fed the amp with an ifi iOne DAC and a laptop on shuffle play during the tube burn in period. The amp gets very warm when run for extended periods and needs good airspace around it to dissipate the heat produced.
Since choice of manufacturer is limited I decided to try and test as many different date ranges as was realistic to see if there was a perceptible difference among them. With the tubes having been in constant production from the early 1960s to today, no shortage of different dates exist. I found tubes as old as 1966 and as new as 1994. I started with the the stock Chinese made tubes, and then swapped in four different pairs of Svetlana made tubes from 1968, 1972, 1978, and 1988, four pairs of Ulyanov made tubes from 1972, 1973, 1975, and 1978 respectively, and finally two pairs of Sovtek from 1980 and 1994 respectively.
Once the tubes had a chance to settle down, I found that all three makes and all years represented are very close in performance. The 6s19P is an extremely linear tube regardless of maker and most of the differences heard throughout my testing were from changing the 12au7 preamp tube and not from swapping the power tubes. If I had to pick favorites amongst the 6s19P, it would be a tie between the early Svetlana made and the newer Ulyanov made tubes. Both are slightly more linear than the stock tube and provide a bit more punch at the low end. I tried using the 7233 with some ugly homemade adapters tube socket with wires soldered to the bottom half of a socket saver with pins 8 & 9 crossed. I used 1 inch 20ga stranded copper wires with silver solder to connect the pin base to the socket. I must admit I was fully expecting it to be erratic at best, but the strange looking contraptions with tubes hanging off the sides like ears performed reasonably well. I found no improvement in sound quality as a result of swapping in the 7233 and if anything, this tube was less linear than the 6s19p so spending more on the 7233 tubes and then having to rig adapters for them is simply not a cost effective proposition.
The biggest changes in sound came with changing out the 12au7 preamp tube and here we have a ton of options. Which tubes suits each user best is going to depend in large measure on the DAC in use, and the users preferred sound signature. The built in Cirrus Logic DAC in the Ta-03s is slightly warm and slightly upper mid/lower treble forward so a tube that augments that is only advisable if you really like those qualities in the sound. For this reason, the Mullard, and Brimar tubes were a bit over the top for me, but if you love a warm sound, they may be exactly the right call for you. Also, when pairing the Ta-03s with an external DAC, these two tubes come into their own. I found when paired to the Burson Swing with its 9038 and slightly clinical sound, those same Brimar and Mullard tubes really came into their own and make the signature a bit less sterile than the stock tube. If using the built in dac, and wanting to tone down the warmth a bit as I did, the Telefunken and Raytheon are good options but the surprise of the test was the Sylvania black plates which actually outperformed the Telefunken at one third the price. The Sylvania had a richer sound than the Telefunken which while very linear sounded a bit thin in the upper mids and treble.
For me the two tubes I would buy to fill the 12au7 socket on the Ta-03s are the old stock Mullard 12au7 or CV491 and the Sylvania black plate (Mid-1960s production). When you want warmth, throw in the Mullard, when you want a bit more linear signature, the Sylvania will provide what you are looking for and both can be had at budget friendly price tags with a bit of looking.
And now we go off the rails, the 12au7 family of tubes has several higher gain options to toy with if one has the desire and I being a inveterate tinkerer had to try them out. The 12at7 can be used in the Ta-03s for those wanting more gain and worked reasonably well but did limit the volume knob to roughly the first 30% of its range unless using something really tough to drive as it simply got too loud to be useful above that. The 12ax7 was not usable for the same reason. It had two volumes off and too loud to be safely used. A bit of swapping 12at7s in and out found that I really liked the Mullard CV4024 and the Phillips 6201 (gold pin) made by Amperex in Holland for warmer sounding tubes and the Telefunken 12at7 diamond marked or the Lorenz 12at7 wide-plate from the early 1960s for a cleaner more linear sound.
I originally thought I had a couple microphonic tubes when using the USB DAC of the Ta-03s but eventually found that when used with the analog input, the noise wasn’t present. A USB cleaner in line with the cable eliminated the noise so it appears the USB input on the Ta-03s is extremely sensitive as the same port and laptop have been used for daily listening with several other USB devices without the need for a USB cleaner. This may be because the amp does draw some power from the USB line rather than stripping it off and using the internal power supply as the device remains present in device manager even with the power turned off. My suspicion is that the USB input board draws current from the USB port rather than being powered by the main, looking at the circuitry seems to confirm this, but as I am not an electrical engineer I’ll stop short of saying this is definitely the case.
If you are interested in a more thorough review of the Ta-03s, check my review of it here.